The overall goal of this proposal is to define and understand the virologic determinants which govern the transmission, fitness, and in vivo replication dynamics of multi-drug resistant HIV-1. The pathway to resistance to individual agents from all three available drug classes has been well characterized. The molecular basis of drug resistance occurs at the level of reverse transcription, when mutations occur and are expressed as amino acid substitutions in altered proteins, such as protease and reverse transcriptase. Ultimately this results in an HIV-1 variant exhibiting a survival advantage in the presence of the strong selective pressure of antiviral agents. As these HIV-1 variants evolve and become more prevalent in the treated community, it becomes critical to determine whether these viruses are being transmitted, and if so in what frequency? To address this issue, the prevalence of resistant HIV-1 transmission will be determined using phenotypic and genotypic analysis of viruses from newly infected individuals. This is of considerable public health importance as the transmission of drug resistant viruses will result in changes in therapeutic practices and will justify pretreatment testing for virus susceptibility to available drugs. It is also critical to understand the determinants of virus fitness and replicative capacity as it relates to transmission. It is hypothesized that by becoming resistant the virus acquires a "handicap" in fitness and replicative capacity. However, it remains unknown whether this affects transmissibility. To determine the effects of resistance-conferring mutations on transmissibility, resistant recombinant viruses will be constructed and their replication kinetics characterized in vitro using a variety of cellular targets which simulate the environment of the mucosal surface at which transmission occurs. Finally, to further understand the viral dynamics of multi-drug resistant viruses, in vivo interventional studies will be performed in individuals to deduce parameters of drug resistant virus replication in vivo.